thoughts on 80W AB1 amps. 2014.
This page is about :-
Need for high quality driver stages for output stages using CFB.
Fig 1. Complete amp schematic 6CG7, 2xEL84, 4xEL34.
Fig 2. B+ Delay, Active Protection, Bias monitoring with LEDs.
Graph 1. Loadlines for one EL34 in PP amp, Ea = 400Vdc,Ia =
Fig 3. OPT details for 75Watt OPT for 4k3 : 4r0 and 7r1.
Why 80W amps?
The 80W is a nominal maximum output power very easily and
cheaply achieved using a quad of octal tubes
such as EL34, 6CA7, 6L6GC, KT66, 807, 5881. One pair of EL34 can
make 40W, so a quad can make 80W
in class AB1 pentode mode with fixed screen Eg2, fixed grid bias
EG1, Ea = +400V, idle Ia = 40mAdc, idle Pda = 16W.
The power wanted for good domestic hi-fi will never be 80W and
will usually average only 1W or less from a single
amp channel. Peaks in music might reach 30W and we want the
first 15W to have very low THD and IMD so this power
should ideally be all class A1 power.
To obtain 80W in AB1 requires a low RLa-a loading better suited
to PA amps or guitar amps where distortion does
not matter. Such loading yields very little initial class A1
power and most power is class B1.
If we insist that all amp power must be only class A1 then amp
efficiency cannot be more than about 45%, which
means that if we use 4 x EL34 with a total Pda at idle of say
80W, then maximum Po is 36W.
And this Po can only occur for ONE load value with load ohms
above or below giving less power. My SE35 is a
classic example of a pure single ended class A amp.
The same 4 tubes could be used to make a PP class A amp giving
same 36W maximum of class A at only ONE
load value, but it could make more power when load ohms are
reduced but give less power when load ohms are
increased. The increased power with lower load will be class
The PP amp working in class AB will produce an initial amount of
pure class A1 before tubes begin working in
class AB. The power between end of class A operation and
clipping is class B power, only one of the pair of PP
tubes produces audio power.
As long as the initial amount of class A1 power is enough to
cover most of what everyone listens to when using an
average of 1W, then 15Watts is usually than enough class A and
higher musical transients such as drumbeats can
be well reproduced by class AB action.
To get adequate class A1 power and good total AB1 power ceiling,
the idle Ea can be higher, Ia lower, Pda lower for
the PP AB amp. In this case, total Pda at idle for 4 x EL34 =
64W. Maximum possible class A is about 30W but only if
RLa-a for the quad = 8k0. RLa-a may be reduced for class AB1
action to about 4k0, and about 13W of pure class A
is possible with about 62Watts total class AB1. It is possible
to get up to 120W in class AB1 with RLa-a below 1k4,
but the THD will be 6 times greater and tubes may easily
overheat, and Eg2 should be +400V.
With 4k0 load for 4 x EL34, and with continuous sine wave at
clipping the EL34 will not overheat, and Eg2 may be
+300V. Other tubes such as 6CA7, KT66, 6L6GC, 807, 5881, may all
be used with same idle conditions for EL34,
although Eg1 may need slight change to obtain the same Iadc.
6550, KT88, KT90, KT120 all can be used but Eg1 will need to be
much more negative than for EL34 for the same
Iadc. These tubes have higher Pda ratings and can idle with
higher Ia of say 55mA to obtain more pure class A.
More class AB power is also available. See 8585 two channel
I believe the best output stage for a quad of PP EL34 has a
fixed Eg2 and CFB windings. It is known as the Acoustical
connection, and was made famous by QUAD, in the Quad-II monobloc
amps. I have used it in my 8585 amp.
The CFB winding should be between 12.5% and 25% of total primary
turns. This always means the maximum drive
Vac needed for each of the output tube grids must be between
50Vrms and 75Vrms, even higher than pure triode, but
still low enough to achieve very low THD if the driver stage
uses medium µ triodes such as EL84 strapped in triode.
The driver stage should not use pentodes in pentode mode.
Other suitables can be EL86 in triode or paralleled 6SN7, 6BL7,
6CG7, 12BH7 and ECC99. EL84 triodes give gain = 18,
EL86 gives 11, and both have Ra < 2k5, so Rout anode to anode
of an LTP driver stage is less than 5k0.
At another page about use of 4 x EL34 for
Mr Zel, I show an input stage using 6DJ8 and a 6BL7
driver, both set up as
a fully balanced differential voltage amp for following EL34
output tubes with 25% CFB. The operation of 6BL7 was
optimised with “bootstrapping” the R supplying Idc between 6BL7
anodes and taps on OPT anode windings.
However, the best optimisation for balanced amp or LTP amp for
driver is done using a CT choke and R between
ends of choke and the two driver anodes.
Fig 1, 75W complete AB1 amp.
In Fig I have 4 x EL34 used with idle Ea = +400V, Ia = 40mAdc,
Pda = 16W. Fixed bias is about –29Vdc,
Eg2 = fixed Vdc = +300Vdc.
Input V1a & V1b is 6CG7 set up as an LTP differential amp
with commoned cathodes connected to constant
current sink using MJE340.
Input signal feeds V1a grid and GNFB feeds V1b grid.
Differential gain about 16 and the stage has good
common mode rejection, and much less THD than any SET for V1
used as V1.
Driver tubes V2 and V3 are a pair of EL84 in triode and in a
balanced amp. C9&10 and R18&R19 give local dc
current FB to best regulate the Ea of each EL84. The cathode
currents of both flow in “tail” R17 3k3 to –100Vdc rail.
The “long” R17 value ensures good balance of the opposite phased
Va from each anode.
I show the OPT with 12.5% CFB, and signal Vrms shown are at
clipping with RLa-a For quad of tubes = 4k3.
Maximum drive Vac to each EL34 grid is about 50Vrms.
The gain of EL84 = 18 approx, so the Vg-g to EL84 must be
5.6Vrms, so V1 6CG7 needs Vg-g = 0.35Vrms,
so Vin with 11dB GNFB is 1.47Vrms.
The amount of CFB can be easily increased to 25% by using OPT
primary layers slightly re-arranged.
Then the Vg to each EL34 may become about 75Vrms, needing Vg-g
at EL84 = 8.4Vrms, and Vg-g at 6CG7
= 0.53Vrms, and if GNFB network R values are left unchanged, Vin
with GNFB = 1.58Vrms, so amount of GNFB = 9dB,
The L1 choke acts to increase the load bringing Idc to EL84
anodes so that the anode load is dominated by 2 x 100k
Rg of EL34. So loading on each EL84 anode is about 50k. 100k is
a low value for Rg but this stops an excessive
positive Vdc across 100k when tubes age when a tiny input grid
current begins to flow. There is no bootstrapping
of anode loads for EL84 with L1 so there is no mild application
of positive FB.
L1 choke with CT has no air gap and provides a high L reactance
in the anode dc feed circuits. It is in fact what is
called “choke feed”, but is in balanced form in this and other
designs at this website. Typical properties of such a
choke have L = 820H at 50Hz, and µ = 5,000, and shunt C = 200pF.
I show 3k3 in series with each anode and ends
of choke winding. Ra of EL84 is about 2k2, and 1/2 Rw = 250r.
Loading of each EL84 is mainly the cap coupled
Bias R for EL34 = 50k. But at very low F and high F the XL and
XC decline towards 0.0 ohms, and lowest load
on EL84 is 3k3 + 250r = 3k5 approx and gain = 12.2. But this
drop in gain is only -3dB at below 1Hz and above
227kHz. The effects of phase shift caused by XL and XC are
avoided. At 1kHz, XL and XL are extremely high,
and load is dominated by 50k and gain = 19 approx. The loading
of the EL84 by the choke becomes quite negligible
for the audio band. You can expect THD at 90Vrms to be <
Choke details for L1 can vary considerably.
For my 300W
amps I used GOSS E&I lams with T = 25mm, S =
32mm. Lams were maximally interleaved with no
air gap, iron µ max = 17,000. Plastic bobbin has centre divider
and 2,500t of 0.2mm Cu dia wire random wound
with slow traversing speed on each 1/2 bobbin, with CT brought
out between the two windings.
Max Idc allowed for each winding = 90mA. Low Rw is not required.
At low signal levels the iron µ = 5,000 assumed. Thus L at 50Hz
= 820H, and reactance XL = 256k. This rises at
higher F to megohms. Shunt C or self capacitance was about
200pF, and XC equals 100k at 8kHz. Because Ra =
2k2, phase shift is avoided. For my 8585 amp I used
same 25T E&I lams but only 10mm stack, and 0.125Cu dia
wire which gave total turns = 8,000. These chokes were then
easily fitted under the chassis. Max Idc allowed each
winding = 37mAdc. With µ = 5,000, at 50Hz, L = 916H, ZL at 50Hz
= 287k. This choke with 8,000 turns can be
carefully random wound with lathe turning at 5 turns per second
and wire fed on from one side of bobbin to other
every 10 seconds, and while lingering at ditches in height to
keep windings level as bobbin is filled. A divided
bobbin makes everything easiest. Just fill one side with 4,000t
and then fill the other. A turn counter is essential.
A series R at each end of winding MUST be used in case EL84
develop a short between anode and 0V to l
imit high current in choke winding which would quickly fuse the
winding. The R value used must limit maximum
Idc to less than Cu wire rating based on 3A/sq.mm of wire
Needless to say, this most wonderful sounding arrangement was
never adopted by any manufacturer because
they always settle for a cheap solution. Details of OPT are
Fig 2. Delayed B+, active protection, bias condition
Fig 2 has several splendid functions to ensure a tube amp will
not cause expensive repairs when something malfunctions
or an owner connects a shorted speaker or speaker cable.
Inrush current delay.
The inrush current at turn on is limited by R21 in mains neutral
line to large PT. This resistance is shunted by relay 1
after about 4 seconds, and when B+ has reached about 2/3 its
maximum level and input current has reduced.
When relay 1 shunts 100r, the input peak current input increases
slightly but is no more than limited initial peak
current at turn on. Without this delay circuit, a fuse of twice
the current value is needed. Q1&Q2 are a darlington pair of
PN100 with high base input resistance.
R1 & C1 have a time constant of 7 seconds, and at about 4
seconds after turn on, the Vdc across C2 has risen high
enough to get Idc flow through 6V8 zener diode to Q1 base, which
turns on quickly enough to turn on relay 1 with
a nice click heard after turn on. R21 100r is in series with
mains neutral line to the large PT primary. The mains
fuse really only protects against catastrophic faults in the amp
PSU, such as electrolytic caps or silicon diodes failing
o become a short circuits, or some winding on PT shorting to 0V
or chassis, or there is a prolonged excessive rise
of mains voltage due to fault in the town supply.
If for any reason the amp is turned off, then back on again in
less than 4 seconds, the delay circuit will again work
to limit inrush current thus avoiding excessive currents which
are all the greater when tubes are still hot from being on.
Bias setting without a voltmeter. There are 4 output tubes in
each channel of this type of amp. The best way to make
any power amp with 4 x EL34 or other octal output tubes is in
monobloc form, complete with its own PSU and protection
circuits. Monobloc use keeps weight to less than about 20kg per
Each channel has its grid voltages adjusted by 10k pots seen in
Fig 1, VR1 to VR4. These are 3W rated wire wound
linear pots. The Fig 2 schematic is can be used to to adjust the
bias pots without a voltmeter, and to be guided by
watching a pair of LEDs for each EL34 while turning the pot
shaft located near LEDs. After adjustment, the grid
bias voltage Vdc stays "fixed" until time comes for any
All output tubes cannot ever be perfectly matched, and will
never stay matched after use, so individual bias adjustments
are needed where there is no "auto-bias" which better suits
class A amps. The yellow and red LEDs should become
equally bright when bias is set correctly. This is easy to see
because a small amount of turn of each pot will make one
of the LED glow brightly, and the other turn off.
Bias pots are best installed under the chassis facing upwards on
sub-plate with 6.3mm dia shaft protruding through
chassis top by 3mm and with a sawn slot on shaft allowing
screwdriver adjustment, or use of kitchen knife by non
technical minded owner. The LEDs should be 5mm dia, facing
upwards and nearly flush with chassis top, and both
pot shaft and 2 LEDs mounted close to each EL34. The arrangement
means there is no need to remove the amp from
its equipment stand, no need to remove a cover or use a
voltmeter, and no danger of amp damage and no risk of
electrocution while probing around the circuitry which has many
points at +400Vdc potential. There is less likelihood
of an owner becoming confused & electrocuted by what he is
doing, or being interrupted by phone call, when his
3yo son wanders over to the amp.
The amp must be turned off before removing or replacing any
tubes. Bias adjustments must be done with amp turned
on but without any signal present. Speakers need not be
connected if the amp is not likely to oscillate.
When plugging in a new set of tubes, all pots should be turned
to mid position before turning amp on. If one tube is to
be replaced, turn off amp, wait 2 minutes, plug in new tube with
bias set to mid position. After turning on, the replacement
tube bias is adjusted and the LED brightness made about the
same. The rest of the tubes should remain biased correctly
if they were before you replaced the tube. When biasing a new
set of tubes, or one tube, DO NOT allow yourself to be
interrupted because some tubes may run too hot with the
incorrect bias setting, leading to possible damage to the tube.
However, if the active protection circuit is used this won't
happen, but having the amp switch off and needing re-setting
during the biasing procedure is a PITA. The adjustments need to
be repeated as tubes warm up until all red & yellow
LEDs have equal brightness after amp has been on for 15 minutes.
Just after turn on all red LED should remain unlit, with all
yellow being bright. This indicates tube current is low, because
tubes have not warmed up. But as current begins to flow after
about 15 seconds, red and yellow LED will flicker and turn
on / off until tube current settles down and LED should all be
lit with about equal brightness. If a yellow LED is unlit,
the red LED should glow brighter, and this means Ia is a little
too high, and if the red LED is unlit, the yellow LED should
glow brighter. Bias pot should be adjusted to equalize LED
brightness. If the bias pot cannot be adjusted to any position
to give equal brightness for LED, and only red LED is lit, or
only yellow is lit, the nearby tube is faulty and MUST be
replaced. But when many tubes fail randomly and early in life,
or after 5,000 hours, they often conduct enough excessive
Idc that the amp protection is triggered. Attempts to reset the
amp by turning off, then back on will fail to fix the problem.
But after turning off, and allowing the amp to cool, then
turning back on, AND while watching which tube is first to have
its red LED turn on brightly, you should be able to work out
which tube is faulty, without trial and error by replacing each
tube in turn with a new replacement.
If you don't like lots of LED, then the use of a voltmeter and
test points is next best thing. You will need to mount
accessible test points to measure the Ek of each EL34 at top of
C8,9,10,11 in Fig 2. The test points should 2mm
chassis mount sockets with 4 red for the four Ek Vdc, and one
black socket connected to 0V rail. The meter
probes may be plugged in while probing each of the four Ek while
turning the pot shaft to get the correct Vdc.
If you don't want LED to set bias and monitor Ek, then you will
not need the 4 differential amps to drive LED, but
you will need the the protection circuit because nothing will
tell you that a tube is conducting too much current or
About tube heat.
It is only natural that amp owners often ignore everything I am
saying here, and never watch the bias condition of
their tubes via the LED, or measure any test point voltages. All
tube amps produce heat while doing nothing at idle.
For each EL34, heater filament power = 6.3V x 1.6A = 10 Watts.
With each EL34 correctly biased, Ia = 40mAdc
and heat generated by anode = 0.04A x 400V = 16W.
The screen dissipates heat = 300V x 3mAdc = 0.9W. So Pda and
Pdg2 heat liberated from EL34 is 16.9W.
With heaters, total heat = 26.9W. The filament power is a fixed
and unavoidable 10Watts. The combined anode
and screen heat must not exceed the EL34 ratings for maximum Pda
+ Pdg2 which is 28W. The heat is called
"anode and screen dissipation." Some EL34 made over last 20
years have Pda+g2 ratings lower, and some higher.
6CA7 were made to be plug a plug in replacements for EL34, and
have same data as EL34, although most 6CA7
many samples withstand higher Pda + Pg2 than original designs of
EL34. With EL34 Pda+Pdg2 = 17W in the amp
here, the EL34 are very happy and temperature is tolerable for
maybe 20 years. If an EL34 conducts say 75mAdc,
its Pd+g2 = 33.75W, and the anode will begin to glow red and
tube is too hot and and tube life is threatened,
and music turns to mud. We wish then that the amp automatically
turn itself off.
LED Bias monitoring.
In Fig 1, each EL34 has a 22r between cathode and the 1/2 CFB
winding which probably has Rw < 4r0.
The correct Ikdc for each EL34 is the sum of Ia and Ig2 =
43mAdc. The Vdc across each 1/2 CFB winding should
be about 0.35Vdc. There should be 0.95Vdc across each 22r in
EL34 cathode circuit. The Ek - 0V should be
approximately 1.25Vdc. In Fig 2, consider K5 cathode point which
is at +1.25Vdc. This Vdc is applied to a low
pass filter formed with R7 3k9 and C8 470uF. This filter removes
the high signal Vac which occurs at the
cathodes which can reach 70Vrms at clipping. During normal
operation we only want to monitor the Ikdc.
Signal Vac below 1.0Hz will have very low amplitude because the
amp response LF pole may be at 7Hz and there
is very little music signal at 1Hz. The R7&C8 offers -20 db
attenuation at 1 Hz, and -51dB at 30Hz, so the normal
signal operation does not affect operation of bias monitoring or
The Vdc at K5 causes some Idc flow in R7 3k9 and to the inputs
to a diode and bjt base input to Q11 buffer and
to each base of each bjt differential amp. The current in R7 3k9
is less than 0.05mA if the hfe of bjts is more than
100. Generic bjts like PN100 costing 10c each are OK to use. So
the RC filter after K5 drives high input impedance
of following devices so it is easier to calculate and predict
Vdc at all points in Fig 2 as I have indicated on the schematic.
Each bjt LTP amp has Vdc input from cathodes on right side bjt
base. Each bjt base on left side of LTP is kept at a
reference voltage of +1.25Vdc. A small Vdc change at any one or
more cathodes is enough to change Idc flow in LEDs.
Automatic turn off with excessive Ikdc. Suppose K5 Vdc rises to
+2.2Vdc because Ia+g2 current in V5 ( Fig 1 ) has
increased to 75mAdc. This rise of Vdc is transferred to Q12 SCR
gate to give +0.68Vdc which is enough to turn on SCR
which turns the whole amp off. Before the amp is turned off, the
rise in Ek also causes a red LED to be turned fully
on with 8mAdc and yellow is turned off. So there is some
indication something is wrong with tubes before automatic
turn off. The connection of a shorted speaker cable or speaker
can cause rapid auto switch off when the owner tries
to turn up the volume with music. The turn off avoids tube
damage. After auto turn off, all yellow LED will remain alight.
The blue “on” LED turns off, and red “fault” LED turns on.
The differential amps in Fig 2 have their gain reduced with 22r
emitter resistors. These R should give less sudden
changes of brightness of LED when setting bias or when
monitoring brightness after bias is set. Grid bias voltage
not need to be adjusted more than once each 3 months. The
correct range of Idc is between say 38mA and 48mA.
The LED will easily tell an owner if tube bias is wrong by more
than +/- 5%. Neither of these very slightly incorrect
conditions will lead to catastrophe, or poorer music quality.
The differential amps require +12Vdc and -12Vdc rails and I have
shown these produced by small 8VA mains transformer
with 15Vac sec giving +/- 19Vdc which are applied to 7812 and
7912 regulator chips which are easier to arrange than
having RC filtering +/- shunt regulation with 12Vdc zener
The -12Vdc rail is VERY important because it provides a near
constant Idc to emitters of bjts in diff amps.
The emitter resistance of Q12 buffer supplies 1.86mA to Q11
emitter follower buffer. At turn on, the gate of SCR should
initially be negative, and slowly rise to -0.18Vdc depending on
Ek of tubes.
The SCR gate has a LPF using R24 4k7 and C14 1uF. If the amp is
turned off automatically all power circuits are
turned off except the 8VA aux trans and its PSU and the devices
it powers. The SCR gate voltage will subside to
ess than 0V when Ek reduce to 0V within seconds but SCR
remains turned on and keeps Relay 2 open and main large
PT turned off. This can be discomforting to an owner, but may
save him much expense later. The amp may be reset
switching amp off then back on after 3 seconds. When the amp is
turned off at switch, power to auxiliary PT is turned off.
Therefore the +12Vdc rail falls rapidly to 0V because of current
through relay 2 and Q12 SCR quickly draining +12Vdc
rail caps. When anode of SCR is below +0.8Vdc, it then turns
off. The time is usually 1second for this to occur.
So the amp can be turned back on and it will try to operate
normally unless the cause of excessive Ek of one or more
Graph 1. Load line analysis for EL34.
Graph 1 shows basic load lines for one EL34. The Ra curves for
EL34 do not have multiple Ra curves for many
values of Eg1 as shown on old data sheets. Such old data sheets
have so many lines they confuse many who
use them. There is only ONE Ra curve you need to know about and
its for Eg1 = 0V and for a specific value of Eg2.
I have FOUR Ra curves here, for 4 different Eg2 between +250Vdc
I have drawn 6 loadlines for various class A and B RLa, all
using idle condition of Ea = +400Vdc, Ia = 40mAdc,
and from there we may calculate expected performance for 4
different Eg2 values, +250V to +400V.
The colored load lines represent loads :-
Dark blue line D-C = B RLa = 666r. Dark blue line E-Q-D =
A RLa = 1k33. These 2 lines are used for RLa-a = 2k66.
I show C on Eg2 = 350V curve and theoretical max Po = 58W AB1.
The theoretical Po assumes Ea and Eg2 should
theoretically remain well regulated but in practice both may
drop 20% with increasing Ia during AB operation, tube
samples may have knee of curve further to right, and winding
losses in OPT may be 10% so at a secondary of a real
amp you might only see 42Watts. Use of UL or CFB can also
slightly move Ra knee right.
Brown line is for B RLa = 1k0 for RLa-a = 4k0, Magenta line is
for B RLa = 1k33, for RLa-a = 5k33, Crimson line is for
B RLa = 2k0, for RLa-a = 8k0.
Black straight line passing through Q is class A RLa = 9k3. This
line shows the class A load for maximum possible
pure class A where max Ia change = +/- 40mApk. For 2 x EL34,
RLa-a = 18k6, and there is no AB operation,
and Po = 14.9W, with each EL34 producing 7.45W.
To allow maximum possible Ea swing with all loads the Eg2 would
need to be +400V. But analysis reveals using
RLa-a = 2k66 gives high THD and tubes will overheat easily. So
let us never ever require the amp to give maximum
possible AB1 Po with 2k66.
Consider Brown line for B RLa = 1k0 for RLa-a = 4k0. The ideal
Eg2 would be +350V because the Ra knee starts
above Ia max at 340mApk. Class AB1 Po max = 57W, class A Po =
3.2Watts for 2 EL34. Using lower Eg2,
clipping would occur at a lower Ea peak swing and lower Po. The
Ra curve prevents voltages extending to the left
of the Ra line or aka the “diode line”. This is because g1 grid
draws grid current and Rin to g1 becomes less than
say 2k0 and the driver amp output clips. EL34 and 6CA7 do not
like class AB2 operating conditions with grids
being forced to go positive. But 6L6GC do not mind, and with Eg2
= +300V, and Ea = +600V, some 80 W is
possible in AB2, but it is unreliable and THD = 13%.
Consider Magenta line for B RLa = 1k33 for RLa-a = 5k33. Eg2
could be as low as +300V. Class AB1 max = 45W,
A1 = 4.2 W.
Consider Crimson line for B RLa = 2k0 for RLa-a = 8k0. Eg2 could
be +250V but better would be +300V, and class A
B max = 33.2W. Class A = 6.4W. The Crimson line represents the
best RLa line for an EL34 for a class AB amp IMHO.
With a quad of EL34, RLa-a for the quad is 1/2 the RLa-a used
for a pair of EL34, and for the quad of tubes RLa-a
could be 4k0. Maximum class AB will be 66W and class A max =
12.8W. Because of low increases of Iadc and Ig2
between first 12W and 66W max, it is easier to rely on large B+
rail caps to keep rail Vdc close to constant during
There is another alternative configuration which some will say
always sounds better. The use of 12.5% CFB is
retained, but instead of having a fixed Eg2 at say +350Vdc,
screens on each side of PP circuit are connected to
UL taps on anode windings for +/-64Vac. See connections 4 and 15
on Fig 1. Series 270r “screen stoppers”
are retained. The EL34 then work with the same gain as for plain
UL with 37% screen taps. The Vac applied to
screens reduces all THD spectra to be similar to triode, with
most reduction of odd number H. The CFB still
is very effective in reducing THD and Rout, but overall the
combined screen FB and CFB gives a better
overall outcome with little change to Vg needed at EL34 grids.
It is not universally true for other tubes such as
6550 which I found gave less THD with fixed Eg2 = +330V and Ea =
+480V. So merely having Eg2 lower than
Ea does a lot to reduce THD. Feel free to make your own
Fig 3. Output transformer details.
Here is a good recipe for a PP OPT meant for 4 x EL34 or other
octal tubes. The winding layers show the
primary has 16 layers in 5 sections with 18 connections. The
numbers give the exact sequence of windings
from one layer to next and arrows show the direction of traverse
width of winding wire across bobbin.
Terminals all should be turret type at least 10mm part and in
rows along each side of bobbin winding.
Avoid having any one primary wire less than 5mm away from any
other wire to prevent arcing if these wires
have high peak Vac or Vdc difference. The primary has 12.5%
Cathode Feedback winding with CT at
8 & 10 and cathodes to 9 & 10. The anode windings have
CT at 7 & 12, and anodes connect to 1 & 18.
Should 25% CFB be used, ALL the central primary section may be
used. Connections along anode windings
may be used for Ultralinear screen taps.
The OPT has Np = 2,368t for RLa-a = 4k3, and sec = 4 parallel
72t windings for 4r0, 3 parallel 96t windings
for 7r1. There could be 2 parallel 144t windings for 16r0, which
probably will never be needed. Fig 1 schematic
shows two ways strap sec windings to give 4k3 : 4r0 or 7r1. This
load matching variability should suit 95% of
hi-fi listeners because 95% of all speakers made are between 3r0
Those that are 16r0 are likely to be old and / or sensitive
types which may be connected to 7r1 and the 40W
available will have lots of class A and sound wonderful. Use of
8r0 speakers with 4r0 strapping will also give
nearly 40W of mainly pure class A. This OPT design is similar to
others I have made for amps producing 100W.
Some previous designs have specified E&I wasteless GOSS 44T
x 62S x 66L x 22H. This is often hard to get,
and this design above uses the more common size of E&I with
50T x 50S x 75L x 25H.
The other possible E&I could be 38T x 100S x 57L x 19H. Afe
= 3800, and Np may be only 1,500t for same
Fsat as 44T x 62S. You may find it a struggle to get all the
wanted turns in the small window, and keep winding
loss % low.
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